Promising next-generation switching devices capable of achieving high voltage, low loss, and high heat resistance include semiconductor devices which include silicon carbide (SiC) or gallium nitride (GaN), which has a large band gap. Such semiconductor devices are expected to be applied to power semiconductor devices, such as inverters.
In particular, as with silicon, SiC-MOSFETs having a MOS structure on the substrate surface have been developed as SiC-based, high-voltage semiconductor devices.
However, SiC and Ga semiconductor devices still have many problems to be solved. A significant one of the problems is that these semiconductor devices have difficulty in having a normally-off device structure.
Particularly, for SiC-MOSFETs, this problem has been found to arise from the fact that it is difficult to form an insulating film of good quality in the MOS structure. The reason is as follows: by paying much attention to formation of a semiconductor layer having good crystallinity, a semiconductor such as SiC which has been selected in view of its expected good crystallinity and good device characteristics, and thereafter an insulating film has been selected in view of its viability by techniques such as thermal oxidation, and the film forming process has been contemplated.